Sleepy aggressive dog - anybody else?

I have a 3-year old brindle pittie girl. She is just an angel, until she gets sleepy, and then she turns into a monster dog. She is naturally protective, but I have socialized her since puppyhood, so she is never agressive to people, or my other dogs and cats. She has never been a fan of other dogs in the neighborhood, but generally just gets bent of shape, and after correction and turning her focus on me, she's fine. However, when she is out at night and sleeping on the couch, if one of the other dogs or my cats comes into her space, she will snap, growl, lunge. My sister and I always immediately correct the behaviour and put her to bed in her crate. I have talked to many pittie owners, and have gotten all kinds of suggestions for aggression, but she isn't aggressive during the day or really at all. It happens only at night, and only sometimes. Has anyone else encountered this, or am I just the lucky one? Oh, and a muzzle is not an option. She becomes psychotic when a muzzle is on. I have talked to vets and behaviourists about this, and everyone suggests I keep an eye on her - well duh. I would just like to be able to have her out at night without any worries. There hasn't been an incident in some time now, but it still happens time and again.

Any advice would be gladly read and appreciated.